1. Field of the Invention
The present invention relates to capacitor discharge welding, and more particularly concerns control of stored voltage and delivered output energy in such welding.
2. Description of Related Art
In capacitor discharge welding, voltage on a capacitor bank is established in order to enable a selected amount of output energy to be delivered to the weld. After loss of energy in the weld transformer, weld head, weld cables and with some leakage from the capacitor, energy actually delivered to the weld site should be sufficient to perform the desired welding operation in an optimum manner. However, as successive welds are made, energy losses and capacitor storage capacity change. When capacitors heat up their storage capacity may change by as much as 50% to 100%. This is particularly true of electrolytic capacitors. Losses in primary and secondary windings of the weld transformer change significantly as resistance of the transformer changes with increased temperature. So, too, the very large diameter weld cables on the weld head itself exhibit resistance increases of as much as 25% as a series of welds is made and the welding apparatus temperature increases.
Further contributing to the problem is the fact that with each successive weld, new parts to be welded are introduced. The new parts are at a lower temperature, not having had a chance to become heated as have the various parts of the welding apparatus. Thus, resistance of the welding parts, which are effectively at a constant relatively low temperature, has a constant resistance value, and the proportions of energy dissipated in the various parts of the apparatus will vary as welding component temperature (not temperature of parts) increases. Increased resistance of the various parts without change in the capacitor bank voltage reduces weld current and degrades the welds being made.
Still further, as the capacitor bank increases in temperature, its capacitance value changes drastically. As is well known, for a constant voltage the amount of stored energy changes in proportion to capacitance. With changes in stored energy and changes in distribution losses through the transformer, weld cables, weld head and the like, there is poor control of the overall weld process.
Another problem manifested in capacitor discharge welding is precision control of a selected capacitor bank voltage. This problem results in part from difficulties in control of the capacitor bank charging circuit. Where silicon control rectifiers (SCR's) are employed as switching devices for charge control, precision shut off of charging current may not be directly available. Charging circuits of this type may employ full wave rectified current through the SCR, which cannot be shut off until current goes to zero. Accordingly, if voltage of the capacitor bank being charged should reach its desired value at a point in the full wave rectified wave form that is not at zero, charging must continue until the wave form reaches its zero value.
To avoid this excessive charge it has been suggested that the charging current be diverted from the capacitor bank when the voltage on the latter has reached its chosen value. Although this arrangement will enable increased precision of control of the voltage on the capacitor bank, the diverted current is wasted and the apparatus is energy inefficient. Moreover, such SCR switched capacitor chargers have an inefficient power factor in that current flows during only 30.degree. to 40.degree. of each 180.degree. of line voltage, and thus the power line current is not used with optimum efficiency. The SCR arrangement effectively draws current, when used with a full wave rectifier for example, during only 2 to 3 milliseconds for each half cycle of line current and fails to utilize the full 8.3 milliseconds for each half cycle that are available.
Accordingly, it is an object of the present invention to provide welding control that avoids or minimizes above mentioned problems.